Word: hominids
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...surprising. Most types of animals--monkeys, whales, cats, apes--come in multiple varieties. As recently as 30,000 or 40,000 years ago, when Homo sapiens was sufficiently evolved to make jewelry and paint hauntingly evocative drawings on cave walls, we shared the planet with a second hominid species, the Neanderthals. And although it seems natural to us that only one species of hominid lives today, it is in fact an exception to nature's way of doing things...
...this lends support to an evolutionary scheme that has been gaining scientific support. Instead of a tree, with a main trunk and a few side branches, hominid evolution is now viewed more as an overgrown bush, lush at every point with multiple competing species. The evidence certainly seems to point that way. Over the past couple of decades, anthropologists have been finding more and more hominid species dating all the way from hundreds of thousands to millions of years ago, many of them overlapping in time. During most of our ancestors' history, it appears, multiple species of humanlike creatures walked...
...multiple hominid species have been the rule all along, there is no reason to think this wasn't the case right from the beginning. Evolution provides plenty of examples in which new types of animals emerge not just as single species but as collections of similar species that share many but not all physical attributes. The rich diversity of finches that Charles Darwin discovered in the Galapagos Islands is perhaps the most famous example...
According to many anthropologists, Brunet's discovery supports the idea that evolutionary diversity was true for hominids as well. "My guess," Wood offers, "is that Sahelanthropus is the first of what will turn out to be a whole handful of apes and apelike creatures living throughout Africa 6 or 7 million years ago." In this bushy model of evolution, even a remarkably modern face might not guarantee that Brunet's new hominid was a direct ancestor of modern humans. Maybe it was just one of several modern-looking hominids that arose at about the same time...
...hominid does eventually upend the conventional wisdom, however, it will raise all sorts of questions. For example, if Sahelanthropus had descendant species that gave rise to H. habilis, asks Harvard's Lieberman, where are they? Nobody knows, moreover, what triggered the emergence of the earliest hominids in the first place. Virtually everyone now agrees that walking upright was the key physical adaptation that set the hominid line in motion. But that adaptation had to have some evolutionary advantage for it to persist. What, exactly, was so great about walking on two legs...